Brennan Johnson opened the scoring before youngsters Will Lankshear, Mikey Moore, Djed Spence and Ashley Phillips all struck.
Postecoglou said afterwards: "Yeah it was a good run-out. It's always good when you play these games and there's a good crowd and a good atmosphere because it feels like a real game sometimes. A friendly game you worry that the competitiveness and intensity isn't there but I thought it was today.
"Both halves were good. We played some decent football. The boys worked hard, no injuries. We're only nine days into pre-season so overall I thought it was a good hit out for us."
On his return to Scotland, the former Celtic boss added: "Yeah, I loved my time here. Such an unbelievable club Celtic. The support I had from day one was unbelievable, but I enjoyed my whole time up here.
"I did, my family did, it was a great two years and I'll always look back with great fondness. Obviously we had great success which helps but aside from that, just the life we lived and the people we met, it will always be a special time in our lives."